Techniques for detecting defects in a steel plate include an ultrasonic test method, a leakage flux inspection method, a magnetic particle inspection method, an eddy-current inspection method, an optical method, and the like.
The leakage flux inspection method is a technique to detect a defect in a steel plate based on leakage flux measured after the density of magnetic flux leaking externally from a surface of a steel plate due to a defect is detected using a magnetic sensor, such as a hall device, converting magnetic flux into an electrical signal.
In FIG. 1, a defect detection apparatus for a steel plate utilizing the foregoing leakage flux inspection method is illustrated.
As illustrated in FIG. 1, defect detection apparatuses for a steel plate of the related art may include a magnetized portion 110 generating magnetic flux to magnetize a steel plate S in a driving direction of the steel plate S and a magnetic sensor array 121 disposed in a width direction of the steel plate S and detecting the density of magnetic flux leaking when created magnetic flux passes through a defect in the steel plate S.
In the meantime, a magnetized portion 210 may include a permanent magnet PM and first and second magnetization poles 111 and 112 extending from opposing sides of the permanent magnet PM. In addition, the magnetic sensor array 121 may be disposed above the first and second magnetization poles 111 and 112 to be centered therebetween (in other words, to allow a distance L1 and a distance L2 to be equal to each other). Leakage flux measured by the magnetic sensor array 121 may be amplified to a predetermined density to be used in detecting a defect in the steel plate S.
In general, the density of leakage flux caused by a defect in the steel plate S may be lowest at the center between the first and second magnetization poles 111 and 112. Conversely, as a defect included in the driving steel plate S approaches the first and second magnetization poles 111 and 112, the density of leakage flux becomes increasingly higher. Thus, in a manner the same as the foregoing apparatus of the related art, in a case in which the magnetic sensor array 121 is disposed at the center between the first and second magnetization poles 111 and 112, there may be a problem in which an amplification rate of leakage flux measured by the magnetic sensor array 121 is required to be increased.
Furthermore, overall defects present not only on a surface of the steel plate S, but also present therewithin are included in leakage flux measured by the magnetic sensor array 121 disposed in the foregoing structure. Thus, there is a problem in which an inner defect present within the steel plate S may not be detected, separately.
Patent Document 1: Korean Patent Laid-Open Publication No. 2013-0068295 (Publication Date: Jun. 26, 2013).